1. Field of the Invention
The present invention is directed generally to a liquid-cooling-type cooling device that uses coolant, and an image forming apparatus including the cooling device.
2. Description of the Related Art
Image forming apparatuses, such as a copying apparatus, a printing apparatus, a facsimile apparatus, or a multifunction peripheral having two or more functions of these apparatuses, have adopted various methods as a method for recording an image of a text, a symbol, and/or the like on a recording medium, such as paper or an overhead transparency film. Widely used among the methods is an electrophotographic method because it enables formation of fine-resolution images at high speeds. Generally, an image forming process performed by an electrophotographic image forming apparatus includes a step of obtaining image information by scanning with an optical device; a step of writing an electrostatic latent image on a photosensitive element based on the scanned image information; a step of forming a toner image on the photosensitive element with toner supplied from a developing device; a step of transferring the toner image formed on the photosensitive element onto a recording medium; and a step of fixing the transferred toner image onto the recording medium.
Meanwhile, it is known that, during the image forming process, heat produced by operations of various devices in the image forming apparatus increases the temperature in the apparatus and yields various detriments. For instance, in the optical device, a scanner lamp for scanning a document and a scanner motor that drives the scanner lamp produce heat; in a writing device, a motor that rotates a polygon mirror at a high speed produces heat. In the developing device, frictional heat is produced when the toner is agitated to be charged; in a fixing device, a heater that thermally fixes the toner image produces heat. When duplex printing is to be performed, a recording medium heated by the fixing device is sent to a conveying path for duplex printing; accordingly, the temperature around the conveyance path increases. When the temperature in the apparatus is increased by these heats, toner softening that can result in production of a poor-quality image or solidification of melted toner that can cause a movable part in the developing device to be locked, thereby causing a breakdown, can occur. A temperature rise can also result in problems including degradation in oil on a bearing and the like, reduction in mechanical useful life of a motor, malfunction of an integrated circuit (IC) on a circuit board, a breakdown, and deformation of a resin part of low heat resistance temperature. Conventionally, to prevent such detriments as discussed above resulting from a temperature rise in an image forming apparatus, cooling has been performed with an air-cooling-type cooling device using a cooling fan, a duct, and the like.
However, in recent years, the number of heat producing members provided in an image forming apparatus has increased with speedup of processes, such as printing. Furthermore, to achieve more compact design, packaging density of components in an image forming apparatus is increasing. This increase in packaging density makes it difficult to optimize airflow design in the image forming apparatus; therefore, heat is likely to be trapped inside the image forming apparatus. Furthermore, in response to the request for energy saving, toners having lower fusing temperatures have been developed to reduce energy consumption during image fixing. When, in particular, such a toner having a lower fusing temperature is used, reducing a temperature rise in an image forming apparatus is ever-more needed. For these reasons, obtaining sufficient cooling effect with a conventional air-cooling-type cooling device is becoming increasingly difficult. Because of this, a cooling device adopting, as a cooling method of a higher cooling capacity, a liquid cooling method has been proposed (see Japanese Patent Application Laid-open No. 2007-24985, for example).
FIG. 12 illustrates the configuration of a general liquid-cooling-type cooling device.
As shown in FIG. 12, a liquid-cooling-type cooling device 900 includes a heat absorbing unit 310 attached to a heat generating portion, or a temperature rise portion 300, a pump 320, a radiator 330, a fan 340, a reservoir tank 350, and piping 360. The piping 360 connects these components and circulates coolant therethrough. The pump 320 circulates the coolant between the heat absorbing unit 310 and the radiator 330 to thereby radiate heat absorbed at the heat absorbing unit 310 through the radiator 330. Moreover, the fan 340 sends an air flow onto the radiator 330, thereby forcibly lowering the temperature of the coolant flowing through the radiator 330. Unlike an air-cooling system, a liquid-cooling system carries heat using liquid refrigerant (coolant) that has a large heat capacity as compared with air; accordingly, a liquid-cooling system has a large heat absorption capacity and is capable of cooling the heat generating portion, or the temperature rise portion 300, effectively.
Generally, copper or aluminum having a high heat conductivity is used as a material of the heat absorbing unit 310 so that the heat absorbing unit 310 has a heat absorption capacity as large as possible. For instance, the heat absorbing unit 310 may be an aluminum or copper block inside which a channel is defined, a member formed by brazing an aluminum pipe to an aluminum plate, or a member formed by connecting a copper pipe to a pipe-like aluminum block with a method, such as diameter expanding and caulking.
Copper or aluminum is also used as a material of the radiator 330 for a similar reason. For instance, the radiator 330 may be constructed by connecting a tube of aluminum, copper, or stainless steel to a corrugated fin of aluminum, copper or stainless steel by brazing or the like.
The piping 360 includes metal pipes and tubes of rubber or resin. Metal pipes are favorable in a point that metal pipes allow reducing evaporation of coolant as compared in a case with tubes of rubber or resin. However, metal pipes cannot be readily bended and are hard to be assembled into devices. For this reason, flexible tubes of rubber or resin are partially used to ensure easy assembling. Meanwhile, when tubes of rubber or resin are to be used, desirably selected are tubes of a material and shape that minimize moisture evaporation and that release a small amount of halogen to prevent corrosion of metal portions contacting the coolant.
As described above, metal materials are used in a heat absorbing unit, a radiator, and the like of a cooling device. In a case in which metal portions of them are made of dissimilar metal materials, what is called galvanic corrosion can occur. Galvanic corrosion is a phenomenon in which, when dissimilar metals in electrical contact are immersed in an electrolytic solution, a difference in ionization tendency between the dissimilar metals based on the standard electrode potentials shown in FIG. 13 develops a potential between the metals in a manner that a noble one (having a lower ion tendency) of the metals acts as a cathode and a base one (having a higher ionization tendency) of the metals acts as an anode; as a result, the base metal of the anode is ionized to become metallic ions and solved in the electrolytic solution, to thus be corroded. Meanwhile, the greater the potential difference between the different kinds of metal materials, the greater the magnitude of an electric current, by which corrosion is promoted.
For instance, in a cooling device including a heat absorbing unit made of a copper block and an aluminum radiator of a corrugated fin type, if the heat absorbing unit and the radiator are electrically connected, an electron conducting pathway is formed therebetween. Meanwhile, coolant is typically an electrolytic solution containing conductive rust inhibiter. Accordingly, an ion conducting pathway is formed via the coolant between the heat absorbing unit and the radiator. For this reason, either one of the metal portions of the heat absorbing unit or the radiator which contact the coolant acts as a cathode, while the other one acts as an anode. Thereby, a galvanic corrosion occurs in which the anode side (the radiator side in this case) elutes into the coolant as metal ion. If the coolant leaks from a corroded part, failure to provide necessary cooling occurs, which can result in production of an anomalous image resulting from a temperature rise. Furthermore, adhesion of leaked coolant to a device, such as an image forming device, can degrade image quality.
Methods of preventing the galvanic corrosion include a method of using a same kind of metal materials to form the metal portions. However, generally, copper is used in the heat absorbing unit to increase cooling capacity, while aluminum is used in the radiator in view of lower cost in many cases; therefore, it is not necessarily possible to select a same kind of metal material in view of performance and cost.
Another conceivable method is to electrically insulate the metal portions from each other to prevent galvanic corrosion. However, in the presence of insulated metal portions, static electricity is likely to build up on the insulated metal portions; therefore, static electricity undesirably builds up on the metal portions in some cases. Examples of a charging unit that electrostatically charges a photosensitive element include: a corona discharge-type charging unit that causes corona discharge by applying a high voltage to a thin metal wire and directs the generated ions onto a surface of a photosensitive element, thereby charging the photosensitive element. Examples further include a charging method of a proximate discharge type in which voltage is applied by bringing a discharge roller having a moderate resistance in contact with or close to the photosensitive element so that the discharge occurs in the vicinity of the contact point or the close point. In particular, in a case of using a charging unit of a corona discharge type or a proximate discharge type as the charging unit that charges a photosensitive element, ions generated from the charging unit are suspended around an image forming device. Therefore, static electricity builds up on the insulated metal portions. The electrostatic charge on the metal portions can exert a negative influence on an image. Moreover, if the amount of electrostatic charge is large, discharge can occur, which poses a problem in terms of safety.